Concrete is the most abundantly used material in the world, second only to water, but the energy needed to make it creates a significant environmental footprint.
Now, MIT engineers have discovered they can make stronger structures using less energy by adding volcanic ash to traditional cement.
According to the engineers’ calculations, it would take 16% less energy to construct a neighbourhood with 26 concrete buildings made using 50% volcanic ash, compared with the energy it takes to make the same structures entirely of traditional Portland cement.
“Cement production takes a lot of energy because there are high temperatures involved, and it’s a multistage process,” said Stephanie ChinChin of MIT’s Department of Civil and Environmental Engineering (CEE).
“That’s the main motivation for trying to find an alternative. Volcanic ash forms under high heat and high pressure, and nature kind of does all those chemical reactions for us.”
Constructing structures, at least in part, from volcanic ash has several advantages.
The rocky material, which lies in ample supply around active and inactive volcanoes around the world, is naturally available; it is typically considered a waste material, as people typically do not use it for any widespread purpose; some volcanic ashes have intrinsic, “pozzolonic” properties, meaning that, in powder form, the ash with a reduced amount of cement can naturally bind with water and other materials to form cement-like pastes.
MIT engineers tested various ratios of concrete to volcanic ash and came to the conclusion that the correct ratio is dependent on the structure being built.
“You can customize this,” says Oral Buyukozturk, professor in MIT’s Department of CEE.
“If it is for a traffic block, for example, where you may not need as much strength as, say, for a high-rise building. So you could produce those things with much less energy. That is huge if you think of the amount of concrete that’s used over the world.”
The engineers found a mixture of finer volcanic ash and Portland cement produced stronger concrete structures, compared with those made from cement alone.
However, the process of grinding volcanic ash down to such fine particles requires energy, which in turn increases the energy that goes into making concrete, also known as the “embodied energy”.
Experiments in the lab revealed a neighbourhood’s infrastructure can be made with considerably less energy if the same buildings are built with concrete made from a cement mixture that is 30% volcanic ash.
“What we’ve found out is that concrete can be manufactured with natural additives with desired properties, and reduced embodied energy, which can be translated into significant energy savings when you are creating a neighbourhood or a city,” Buyukozturk says.